5 Answers
5

I bet this is very subjective and fuzzy but by my definition it would be something like this:

up to about 10 am is Morgen

until noon (12 o'clock sharp) it's Vormittag

12 till 1 or maybe 2 pm is Mittag

until 5 or 6 pm it's Nachmittag

after that it's Abend until about 10 pm

after that it's Nacht

I think especially the transitions from Morgen to Vormittag and Nachmittag to Abend are very subjective because they are caused by each individual's work day. So for a baker that starts working at 4 am this could be Morgen; for others this might still be Nacht (e.g. for me, I get up at about 7 am).

People who work late (say until 8 pm) might say their Abend starts at that time.

musiKk already gave a good explanation. I think, however, that the definition might totally depend on your day (since it's really very subjective), especially on your Essgewohnheiten (eating habits):

From waking up until about breakfast time, it's Morgen (maybe for another hour)

Lunch time is Mittag (hence Mittagessen)

Evening meal time starts Abend (hence Abendbrot / Abendessen)

Between breakfast and lunch, it's Vormittag

Between lunch and evening meal, it's Nachmittag

Night starts when you go to bed (or some time before), and ends when you wake up. Since the Morgen is the first part of the day, this also means that the new day also starts after you woke up - and not right after midnight.

When you get home from work, it is Feierabendzeit - note that there's also Abend in the word.

This leads to situations like this: When I come to work, it's Morgen for me - but some people greet me with Mahlzeit - even though by other definitions, like the one by musiKk, this is Vormittag ;) On the other hand, this doesn't matter, since we mostly say Moin all day.

The consequence of this definition is that for example some people have Mittag at 11 am, while others have it at 2 pm. For some, the Morgen ends at 7 am, for others it ends at 10 am. This seems to match usage of these words for many people (from my observations).

All these time expressions can be modified with früh or spät. Später Morgen and früher Vormittag blend into each other, später Vormittag and Mittag, and so on.